1. Field of the Invention
The present invention relates to a positioning structure of an image forming apparatus.
2. Description of the Related Art
Conventionally known is an image forming apparatus for forming a color image on a image recording medium by a plurality of ink head groups ejecting respective color ink fluids, i.e., black (K), cyan (C), magenta (M) and yellow (Y), onto the image recording medium.
Such an image forming apparatus equips itself with a transport mechanism for transporting an image recording medium, which is commonly called a belt platen, and a recording unit equipped with four respective color ink head group which are fixed at a predetermined interval in the transport direction of the image recording medium and above the aforementioned belt platen for ejecting the respective color ink fluids, i.e., black (K), cyan (C), magenta (M) and yellow (Y) as noted above.
A configuration is such that the belt platen is an endless band belt, is spanned between a drive roller and a driven roller, is placed with an application of a tension from the lower inside part of the belt by using a tension roller which also has the function as belt walk adjustment mechanism and is set to move rotationally in a prescribed moving direction by the driving rotation of a drive roller and the driven rotation of a driven roller.
The above described platen belt brings in an image recording medium and also transports the aforementioned image recording medium toward the downstream of a plurality of ink head groups along with the movement of the belt while suction-holding the image recording medium. The ink head group ejects ink on the image recording medium transported downstream for forming an image. The image recording medium forming the image thereon is then transported by the belt platen for exiting to the outside of the apparatus.
Such an image forming apparatus is configured to equip itself with a common reference part for the belt platen and recording unit in the main body of the apparatus so as to position the belt platen and recording unit by making them respectively contact with the reference part (e.g., refer to a laid-open Japanese patent application publication No. 2004-161477.) Also known is an image forming apparatus for forming a toner image on an image recording medium a lá sheet of paper transported by a belt platen by a recording unit of an electrophotography system.
Such an image forming apparatus is configured to equip itself with a roller pair (called a “registration roller pair” hereinafter) on the carry-in side of an image recording medium of the belt platen (i.e., the upstream side of the transport direction of an image recording medium) for stopping temporarily to stop transporting the image recording medium supplied from a paper supply apparatus and resuming a rotation synchronously with the image forming timing to introduce the image recording medium to the belt platen in order to regulate and correct an inclination of the image recording medium in the direction of transportation (e.g., skew) and synchronize with an image forming timing of the recording unit.
Such an image forming apparatus is capable of carrying an image recording medium whose transported position is corrected by the registration roller pair into the belt platen in a correct transported position and suctioning the image recording medium onto the belt by a suction force of the belt platen.
Then, the recording unit transfers a toner image onto the image recording medium transported by the belt platen, a fusing apparatus fuses the toner image on the image recording medium by transporting the image recording medium which has been transferred with the toner image into the fusing apparatus, and the exit roller pair succeeds the transportation for ejecting to the outside of the image forming apparatus (e.g., refer to a Japanese registered patent No. 2731963).
Incidentally, while the recording unit of the above noted Japanese registered patent No. 2731963 comprises only one photoreceptor, what exists also, however, is a tandem type electrophotographic image forming apparatus in which four photoreceptors for black (K), cyan (C), magenta (M) and yellow (Y) are serially placed a lá multiple stages in a predetermined interval along the transport direction. The difference between these two are the numbers of photoreceptors, one versus four, otherwise the other configurations are almost the same.
By the way, in order to accomplish a high accuracy image quality, a transport direction of an image recording medium regulated for direction (i.e., a skew is corrected) by the registration roller pair matches with a transport direction of the image recording medium by the belt platen, and also nozzle rows of a ink head group (which is the rotation shaft of a photoreceptor body in an electrophotographic image forming apparatus) are installed respectively in parallel with one another in the direction perpendicular to the transport direction of the image recording medium, thereby securing a printing accuracy.
Otherwise, if the above described positional relationship is not stable in other words, a displacement among each color pixel occurs so as to bring about a color shift in a formed image in overlapping four colors being formed on the image recording medium, thus resulting in failing to secure a high accuracy printing.
A cause for such an unstable positional relationship or a displaced one among the registration roller pair, belt platen and recording unit includes the fact that a distortion in the apparatus body frame occurs when moving the image forming apparatus during transportation for example, causing a distortion in the positional interrelationship among the registration roller pair, belt platen and recording unit, resulting in the transport direction of the image recording medium by the belt platen shifting or the position of the ink head group (that is the position of a photoreceptor in an electrophotographic image forming apparatus) which is fixed onto the recording unit shifting vis-à-vis the transport direction of the image recording medium by the belt platen.
Either of the above noted reference documents, however, does not disclose a comprisal, mechanism, method, et cetera, for the purpose of reducing an influence of a distortion of an apparatus body frame to a positional interrelationship among the registration roller pair, belt platen and recording unit as much as possible, nor is there a description as to suggest any.